The present invention relates to acicular metal iron fine particles containing carbon, a process for preparing the same, magnetic coating composition and magnetic recording medium containing the same.
Conventional acicular metal iron fine particles are much superior in magnetic characteristics to acicular fine particles of iron oxide such as .gamma. --Fe.sub.2 O.sub.3, and are useful as a magnetic material for a magnetic recording medium such as a magnetic recording tape.
However, acicular metal iron fine particles have a drawback of being low in corrosion resistance because the the surfaces thereof are constituted by metal. In order to improve corrosion resistance, the surfaces of the metal iron fine particles are treated with oxygen or air to form thereon a non-magnetic or a low-magnetic oxide layer to prevent deterioration of magnetic properties with a lapse of time. However, this method sacrifices magnetization in an amount corresponding to that amount of the metallic iron which is covered with the non-magnetic oxide and usually 20 to 30% of an inherent saturation magnetization per unit weight of the metal iron are sacrificed when including the weight of a sintering-preventing agent which is added during a preparation process.
Another method of improving corrosion resistance is proposed to form a graphite film on a surface of the magnetic metal powder (JP-A-2-69904). In this method, a minute layer of graphite film is formed by the catalytic activity on the surface of the metal powder. However, saturation magnetization per unit weight is also sacrificed since the graphite is non-magnetic and a large amount of sintering-preventing agent is required to retain the metal in powder form.
Further, a magnetic metal powder which contains carbon, in the form of substantially metal carbide and is obtained from a transition metal carbonyl compound, is also proposed in JP-A-2-69903, but this method has a drawback of being expensive in its starting material. In addition, acicular particles are obtained by placing non-acicular primary particles in a magnetic field but the resulting particles generally have a large shape, which may be difficult to coat and oriented, and are unsuited for use in a magnetic recording medium.
Acicular iron carbide fine particles are known as a magnetic material which is excellent in corrosion resistance. It is known that acicular particles containing iron carbide are prepared by contacting acicular iron oxyhydroxide or acicular iron oxide with CO or a mixture of CO and H.sub.2 at 250.degree. to 400.degree. C. and that the resulting particles are stable chemically, have a high coercive force and are useful as a magnetic material for magnetic recording medium. (For example, refer to JP-A-60-71509, 60-108309, 60-127212 and 60-155522. The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, the acicular iron carbide fine particles are inferior in saturation magnetization per unit weight to acicular metal iron fine particles.